My invention relates to a digital tone generator for a radio transmitter, and particularly to a digital adder for changing the phase of the digitally generated tone.
In some frequency modulation radio communication systems, a tone of audio frequency (below that usually reproduced by a radio receiver) is transmitted to activate only a selected receiver or receivers, and thus provide privacy and prevent undesired messages from being reproduced by other receivers during the transmission. This arrangement is sometimes referred to as a continuous tone controlled squelch system (hereafter CTCSS) or a channel guard system. If the transmitted carrier is cut off when the transmission ends (by unkeying the microphone), then a burst or tail of noise will be heard at the activated receiver until the receiver noise squelch circuit takes over and mutes the receiver. In order that this tail of noise be blocked or eliminated, some frequency modulation communication systems use an arrangement that continues transmission of the carrier for a short period (such as 175 milliseconds) after the microphone is unkeyed. During this period while the carrier continues to be transmitted, the phase of the tone is shifted. Upon receipt of the tone with a phase shift, the activated receiver triggers a circuit to mute itself. During the time needed for the receiver to mute itself, the presence of the carrier prevents noise from being heard. Thereafter the receiver remains muted until a carrier with the proper tone is received again. Since digital circuits are being used increasingly because of their small size and versatility, digital tone generators are being used so that a tone of any desired frequency can be easily selected. Thus, there is a need for an arrangement to shift the phase of a digitally generated tone.
Accordingly, a primary object of my invention is to provide a new and improved arrangement for shifting the phase of a digitally generated tone transmitted by a radio transmitter.
Another object of my invention is to provide a digital adder that responds to keying of a radio transmitter and changes the phase of a digitally generated tone transmitted by the transmitter.